When your being interviewed for a job be prepared to answer a classic interview question regarding your strengths. This is an invitation to talk about your skills…perhaps its analytical thinking; or, maybe organization…wait-just-a-minute, ahh...wrong topic! The strength I am referring to matters when one is considering the strength of a material. So, do you know, or did you ever consider “Specific Strength” when deciding what material to use in a design? No…well, you should have! You may have a good design; but, is it the best design…. If not, consider adding Specific Strength to your decision matrix before making that all too critical material choice.
So, what is Specific Strength?
- Specific Strength is a ratio equal to the ultimate tensile strength of a material divided by the material’s density. Another term that is used to describe specific strength is breaking length; also known as self-support length.
- Specific Strength can be visualized as the maximum length a vertical column of material can be (assuming a constant cross-section) that could be suspended under its own weight if fixed at one end only but will not break.
Specific Strength calculations are straightforward. Let say you want to use steel in your design. First you need to determine the ultimate tensile strength of let's say steel. This first requires a simple lookup operation using a reference such as MMPDS for example. The ultimate tensile strength for Steel (for this example) is equal to 60,900 psi and the density is equal to 0.29 lb/in3. To calculate the Specific Strength, simply divide 60,900 psi by 0.29 lb/in3 to get 210,000 inches. Now, let's look at Aluminum. The values for Aluminum are as follows: the density is equal to 0.098 lb/in3 and the ultimate tensile strength is equal to 45,000 psi. The specific strength for Aluminum is equal to 415,384 inches. Comparing that value with steel’s specific strength and one quickly realizes the benefit that Aluminum has over steel...assuming, in this case, that one of your goals is weight savings without any strength critical issues.
The units for Specific Strength are length…but what is the area of the column? Well, the area is already defined in the tensile strength value (psi). Remember for Steel the strength is equal to 60,900 psi, it is here that the cross -section is clearly observed...per “Unit Area”.
Hey! What about Carbon fiber!? Ok…in this example will use Figure 2 below. According to the chart, High strength carbon epoxy has a Specific Strength of about 1,600,000 inches. A factor of 7.6 times the Specific Strength of Steel…perhaps this is one of the more obvious reasons why composites are so popular in the auto and Aerospace industry.
Figure 2 Specific Strength
So what is this Specific Strength anayway...what does the length really mean? Well, lets look at the Breaking Length chart below. Looking at Titanium for example, we see that the Breaking Length for Titanium is 29.4 kilometers (or 29,400 meters). This length means that if you were to construct a column equal to 29,400 meters in length (go ahead, I dare you to make it...go ahead), you could suspend that column from one end and it would not break. However, if you were to increase the length to 29,401 meters, the column would fracture catostrophically.
Breaking Length Chart
Glass has a breaking length of 83 miles. Obviously, glass is very strong; however, like the example above regarding Titanium, if you were to increase its length ever-so slightly, the glass would break and shatter into a million pieces (well, maybe just...999,999 pieces). What a mess!
Well...that concludes this article, hopefully you have either discovered a new appreciation for Specific Strength or, have been reminded of how such a simple ratio like Specific Strength can have such a big impact on your design. Remember to never lose sight of what your Specific Strength is…because as an engineer, you should be interested in materials that have high strength-to-weight ratios that translate into greater weight savings and improve performance to yield significant gains in overall system efficiency. As always, thanks fore visiting ABDmatrix.com...comments are welcomed.